The present invention relates generally to processing of electromagnetic signals, and, in particular, to detection of excessively high interference signal levels during reception of global navigation satellite system signals.
Global navigation satellite systems (GNSSs) can determine positions with high accuracy. Currently deployed global navigation satellite systems are the United States Global Positioning System (GPS) and the Russian GLONASS. Other global navigation satellite systems, such as the European GALILEO system, are under development. In a GNSS, a navigation receiver receives and processes radio signals transmitted by satellites located within a line-of-sight of the receiver. The satellite signals comprise carrier signals modulated by pseudo-random binary codes. The receiver measures the time delays of the received signals relative to a local reference clock or oscillator. Code measurements enable the receiver to determine the pseudo-ranges between the receiver and the satellites. The pseudo-ranges differ from the actual ranges (distances) between the receiver and the satellites due to various error sources and due to variations in the time scales of the satellites and the receiver. If signals are received from a sufficiently large number of satellites, then the measured pseudo-ranges can be processed to determine the code coordinates and coordinate time scales at the receiver. This operational mode is referred to as a stand-alone mode, since the measurements are determined by a single receiver. A stand-alone system typically provides meter-level accuracy.
The accuracy, precision, stability, and reliability of GNSS measurements can be improved by differential navigation measurements relative to a reference base station, by measurements on more than one carrier frequency, and by measurements of the phases of the satellite carrier signals. A differential navigation system that computes positions based on real-time carrier signals, in addition to the code pseudo-ranges, is often referred to as a real-time kinematic (RTK) system. An RTK system can provide accuracies on the order of 1-2 cm.
The signal levels of GNSS signals, however, are relatively low, and interference signals can degrade or disrupt GNSS measurements.